<script>
fetch("http://example.test:8000/xmlhttprequest/resources/access-control-allow-lists.php?origin=*", {method: "POST"})
    .then(() => {
      window.opener.postMessage('FAIL: Fetch API succeeded unexpectedly', '*');
    }, e => {
      window.opener.postMessage('DONE', '*');
    });
</script>
